A spoolable pipe in common use is steel coiled tubing which finds a number of uses in oil well operations. For example, it is used in running wireline cable down hole with well tools, such as logging tools and perforating tools. Such tubing is also used in the workover of wells, to deliver various chemicals downhole and perform other functions. Coiled tubing offers a much faster and less expensive way to run pipe into a wellbore in that it eliminates the time consuming task of joining typical 30 foot pipe sections by threaded connections to make up a pipe string that typically will be up to 10,000 feet or longer.
Steel coiled tubing is capable of being spooled because the steel used in the product exhibits high ductility (i.e. the ability to plastically deform without failure). The spooling operation is commonly conducted while the tube is under high internal pressure which introduces combined load effects. Unfortunately, repeated spooling and use can cause fatigue damage and the steel coiled tubing can suddenly fracture and fail. The hazards of operation, and the risk to personnel and the high economic cost of failure resulting in down time to conduct fishing operations, typically forces the product to be retired before any expected failure after a relatively few number of trips into a well. The cross section of steel tubing may expand during repeated use, resulting in reduced wall thickness and higher bending strains with associated reduction in the pressure carrying capability. Steel coiled tubing presently in service is generally limited to internal pressures of about 5000 psi. Higher internal pressure significantly reduces the integrity of coiled tubing so that it will not sustain continuous flexing and thus severely limits its service life.
It is therefore desirable to provide a substantially non-ferrous spoolable pipe capable of being deployed and spooled under borehole conditions and which does not suffer from the structural limitations of steel tubing, and which is also highly resistant to chemicals. Such non-ferrous spoolable pipe often carries fluids which may be transported from the surface to a downhole location, as in the use of coiled tubing, to provide means for treating formations or for operating a mud motor to drill through the formations. In addition, it may be desirable to transport devices through the spoolable pipe, such as through a coiled tubing bore to a downhole location for various operations. Therefore, an open bore within the spoolable pipe is essential for some operations.
In the case of coiled tubing, external pressures can also be a major load condition and can be in excess of 2500 psi. Internal pressure may range from 5,000 psi to 10,000 psi in order to perform certain well operations; for example, chemical treatment or fracturing. Tension and compression forces on coiled tubing are severe in that the tubing may be forced into or pulled from a borehole against frictional forces in excess of 20,000 lbf.
For the most part, prior art non-metallic tubular structures that are designed for being spooled and also for transporting fluids, are made as a hose, whether or not they are called a hose. An example of such a hose is the Feucht structure in U.S. Pat. No. 3,856,052, which has longitudinal reinforcement in the side walls to permit a flexible hose to collapse preferentially in one plane. However, the structure is a classic hose with vulcanized polyester cord plies which are not capable of carrying compression loads or high external pressure loads. Hoses typically use an elastomer such as rubber to hold fiber together, but do not use a high modulus plastic binder such as epoxy. Hoses are generally designed to bend and carry internal pressure, but are not normally subjected to external pressure or high axial compression or tension loads. For an elastomeric type material, such as that often used in hoses, the elongation at break is so high (typically greater than 400 percent) and the stress-strain response so highly nonlinear, it is common practice to define a modulus corresponding to a specified elongation. The modulus for an elastomeric material corresponding to 200 percent elongation typically ranges from 300 psi to 2000 psi. The modulus of elasticity for plastic matrix material typically used in a composite tube tends to range from about 100,000 psi to about 500,000 psi or greater, with representative strains to failure of from about 2 percent to about 10 percent. This large difference in modulus and strain to failure between rubber and plastics, and thus between hoses and composite tubes, is part of what permits a hose to be easily collapsed to an essentially flat condition under relatively low external pressure and eliminates the capability of the hose to carry high axial tension or compression loads, while the higher modulus characteristic of the plastic matrix material used in a composite tube tends to be sufficiently stiff to transfer loads into the fibers and thus resist high external pressure, axial tension, and compression without collapse. Constructing a composite tube to resist high external pressure and compressive loads may include the use of complex composite mechanics engineering principles to ensure that the tube has sufficient strength. It has not been previously considered feasible to build a truly composite tube capable of being bent to a relatively small diameter, and be capable of carrying internal pressure and high tension and compression loads in combination with high external pressure requirements. Specifically, a hose is not expected to sustain high compression and external pressure loads.
In operations involving spoolable pipe, it is often necessary to make various connections, such as to interconnect long sections or to connect tools or other devices into or at the end of the pipe string. With steel coiled tubing, a variety of well-known connecting techniques are available to handle the severe loads encountered in such operations. Threaded connections as well as welded connections are often applied and meet the load requirements described.
Grapple and slip type connectors have also been developed for steel coiled tubing to provide a low profile while being field serviceable. However, these steel tubing connectors tend not to be applicable to modern composite coiled tubing. One such connector is shown in U.S. Pat. No. 4,936,618 to Sampa et al., showing a pair of wedge rings for making a gripping contact with the coiled tubing. The PETRO-TECH Tools Incorporated catalog shows coiled tubing E-Z Connectors, Product Nos. 9209 to 9211 that are also examples of a slip type steel coiled tubing connector.
Another connector for reeled thin-walled tubing is shown in U.S. Pat. No. 5,156,206 to Cox, and uses locking slips for engaging the tubing in an arrangement similar to the Petro-Tech connector. U.S. Pat. No. 5,184,682 to Delacour et al. shows a connector having a compression ring for engaging a rod for use in well operations, again using a technique similar to a Petro-Tech connector to seal against the rod.
These commercial coiled tubing connectors would not be expected to seal properly to a composite pipe, partially because of circumferential deformation of the pipe inwardly when the connector is on the composite pipe, and also because the external surface of a composite tube or pipe tends not to be as regular in outer diameter tolerance as a steel hose.
U.S. Pat. No. 4,530,379 to Policelli teaches a composite fiber tubing with a structural transition from the fiber to a metallic connector. The fibers may be graphite, carbon, aramid or glass. The FIG. 4 embodiment can be employed in a fluid conveyance pipe having bending loads in addition to internal pressure loads, and in structural members having bending and axial stiffness requirements.
While many connectors designed for application to elastomeric hoses and tubes, such as shown in U.S. Pat. No. 3,685,860 to Schmidt, and U.S. Pat. No. 3,907,335 to Burge et al., sealing to these hoses is substantially different in that the hose body itself serves as a sealing material when pressed against the connecting members. A composite pipe would be considered too rigid to function in this way. U.S. Pat. No. 4,032,177 to Anderson shows an end fitting for a non-metallic tube such as a plastic tube and having a compression sleeve and a tubing reinforcing insert, but again appears to rely on the tube being deformable to the extent of effecting a seal when compressed by the coupling.
Another coupling for non-metallic natural gas pipe is shown in U.S. Pat. No. 4,712,813 to Passerell et al., and appears to show a gripping collet for engaging the outer tubular surface of the pipe and a sealing arrangement for holding internal gas pressure within the pipe, but no inner seals are on the pipe and seals cannot be changed without disturbing the gripping mechanism.
U.S. Pat. No. 5,351,752 to Wood et al. appears to show a bonded connector for coupling composite tubing sections for pumping a well. The composite tubing has threaded fittings made of composite materials which are bonded to the tubing.
Often, connectors (e.g., interconnects, flanges, and other components for connecting the end of tubing to other elements) are formed from a single piece of bar stock. Because of the dimensional differences between different sections (e.g., a narrower section to fit in the tubing and a thicker section to extend beyond the tubing), the bar stock must be machined down from the thickest dimension, often resulting in a sizable loss of material. Additionally, forming the connector from a single piece of bar stock limits the connector to a single material. This may further increase material costs when a more expensive material is required for only certain portions of the connector but is used for the entire connector.